1. Field of the Invention
This invention relates to a method for purging of, and debris removal from, surfaces being subjected to laser beams for the purpose of creating holes. In one aspect, this invention relates to a method for purging of, and removing debris from, holes produced by laser drilling. In one aspect, this invention relates to a method for purging of, and removing debris from, wellbores and wellbore perforations.
2. Description of Related Art
When a laser beam is used to make a hole in solid materials through thermal and/or chemical spallation, melting, or material weakening, debris is created which must be removed to expose a fresh surface to which the laser beam can be applied, thereby increasing the effectiveness of hole formation, and increasing the rate of penetration by the laser beam. FIG. 1 shows a hole 10 created by a laser beam 11 and producing debris 12 to be removed from the hole. The material or debris resulting from the application of a laser beam to a hole wall and/or hole bottom may be vapor, molten, or solid particles or chunks. Some of the material or debris may stay attached to the hole wall and/or hole bottom surface while some of the material or debris may separate therefrom. The debris, whether loose or stuck to such surfaces, absorbs at least some of the laser energy, thereby preventing it from reaching the underlying surface to which the laser energy is being applied and reducing the effectiveness of the laser energy. If the material or debris is not removed continuously or intermittently, it may continue to absorb portions of the laser energy, resulting in the formation of molten material that sticks to the hole wall and/or bottom surfaces and further adversely impacting the hole shape, the hole surface, and the rate of penetration. Thus, it is desirable to promptly remove the debris from the hole so as to maximize laser energy transmission to the surface to which the laser beam is being applied and to expose fresh surface for treatment. It is also desirable to remove the debris using means, such as a gas jet impinging on the lased surface, that does not absorb laser energy.
One commonly employed approach is to use a purge gas nozzle 13 to provide a straight gas jet 14 of sufficient flow and velocity into the hole to impinge on the lased surface, loosen any sticking debris, and remove the debris 15 from the hole together with any self loosened debris as shown in FIG. 2. Another approach is to use a mechanical nozzle 16 to vacuum the surface being lased as shown in FIG. 3. The first approach is effective in relatively shallow holes and in holes with an opening near the back to allow exiting of at least a portion of the purge gas; but with substantially closed end or deeper holes with a depth to diameter ratio of 5 or greater, this technique becomes less effective as the inherently expanding gas jet 14 loses momentum, traps gases in the hole and is unable to penetrate and impinge on the lased surface near the end of the hole as shown in FIG. 4.
Tests have shown a maximum purge gas jet penetration of about 14 inches in a 2 inch diameter hole, regardless of the jet velocity at the nozzle exit. An approximately 2 inch diameter by 24 inch deep, close-ended hole was filled with fine sand and an approximately ½ inch diameter air nozzle was positioned proximate the opening of the hole with its axis substantially parallel to the hole axis. Tests were carried out using air with nozzle pressures of up to 100 psig. The tests showed that, even at 100 psi pressure, the air was not able to lift the fine sand out of the hole beyond the first 14 inches.
Fluid dynamics modeling carried out with relatively high nozzle pressures showed similar results. To obtain the maximum achievable depth, one approach is to progress the purge gas nozzle into the hole as the hole gets deeper. However, this approach results in shadowing of the laser beam, heating of the nozzle, and a generally non-symmetrical hole. Similar issues are encountered when attempting to use a vacuum which requires that the tip of the vacuum be placed close to the surface being vacuumed. As is well known, the effectiveness of vacuuming drops rapidly as the distance between the surface being vacuumed and the nozzle tip increases. In addition, in difficult to access remotely controlled applications, such as when making holes in an energy extraction wellbore, it is difficult to maneuver the nozzle to progress into a hole in the sidewall of the wellbore, and especially deep into the hole.